Subject: Re: [RFC PATCH] repo-settings: set defaults even when not in a repo

From: Derrick Stolee <[email-masked]>
Date: Wed, 23 Mar 2022 15:22:13 -0400

On 3/23/2022 2:03 PM, Josh Steadmon wrote:
> prepare_repo_settings() initializes a `struct repository` with various
> default config options and settings read from a repository-local config
> file. In 44c7e62 (2021-12-06, repo-settings:prepare_repo_settings only
> in git repos), prepare_repo_settings was changed to issue a BUG() if it
> is called by a process whose CWD is not a Git repository. This approach
> was suggested in [1].
> 
> This breaks fuzz-commit-graph, which attempts to parse arbitrary
> fuzzing-engine-provided bytes as a commit graph file.
> commit-graph.c:parse_commit_graph() calls prepare_repo_settings(), but
> since we run the fuzz tests without a valid repository, we are hitting
> the BUG() from 44c7e62 for every test case.
> 
> Fix this by refactoring prepare_repo_settings() such that it sets
> default options unconditionally; if its process is in a Git repository,
> it will also load settings from the local config. This eliminates the
> need for a BUG() when not in a repository.

I think you have the right idea and this can work.
 
> -	/* Booleans config or default, cascades to other settings */
> -	repo_cfg_bool(r, "feature.manyfiles", &manyfiles, 0);
> -	repo_cfg_bool(r, "feature.experimental", &experimental, 0);
> +	if (r->gitdir) {
> +		/* Booleans config or default, cascades to other settings */
> +		repo_cfg_bool(r, "feature.manyfiles", &manyfiles, 0);
> +		repo_cfg_bool(r, "feature.experimental", &experimental, 0);

However, this giant if block is going to be a bit unwieldy,
especially as we add settings in the future.

Ignoring whitespace, this is your diff:

diff --git a/repo-settings.c b/repo-settings.c
index b4fbd16cdc..d154b37007 100644
--- a/repo-settings.c
+++ b/repo-settings.c
@@ -17,9 +17,6 @@ void prepare_repo_settings(struct repository *r)
 	char *strval;
 	int manyfiles;
 
-	if (!r->gitdir)
-		BUG("Cannot add settings for uninitialized repository");
-
 	if (r->settings.initialized++)
 		return;
 
@@ -28,6 +25,7 @@ void prepare_repo_settings(struct repository *r)
 	r->settings.core_untracked_cache = UNTRACKED_CACHE_KEEP;
 	r->settings.fetch_negotiation_algorithm = FETCH_NEGOTIATION_CONSECUTIVE;
 
+	if (r->gitdir) {
 		/* Booleans config or default, cascades to other settings */
 		repo_cfg_bool(r, "feature.manyfiles", &manyfiles, 0);
 		repo_cfg_bool(r, "feature.experimental", &experimental, 0);
@@ -50,16 +48,6 @@ void prepare_repo_settings(struct repository *r)
 		repo_cfg_bool(r, "core.multipackindex", &r->settings.core_multi_pack_index, 1);
 		repo_cfg_bool(r, "index.sparse", &r->settings.sparse_index, 0);

(Adding the if)

-	/*
-	 * The GIT_TEST_MULTI_PACK_INDEX variable is special in that
-	 * either it *or* the config sets
-	 * r->settings.core_multi_pack_index if true. We don't take
-	 * the environment variable if it exists (even if false) over
-	 * any config, as in most other cases.
-	 */
-	if (git_env_bool(GIT_TEST_MULTI_PACK_INDEX, 0))
-		r->settings.core_multi_pack_index = 1;
-
 		/*
 		 * Non-boolean config
 		 */
@@ -93,6 +81,17 @@ void prepare_repo_settings(struct repository *r)
 			else
 				die("unknown fetch negotiation algorithm '%s'", strval);
 		}
+	}
+
+	/*
+	 * The GIT_TEST_MULTI_PACK_INDEX variable is special in that
+	 * either it *or* the config sets
+	 * r->settings.core_multi_pack_index if true. We don't take
+	 * the environment variable if it exists (even if false) over
+	 * any config, as in most other cases.
+	 */
+	if (git_env_bool(GIT_TEST_MULTI_PACK_INDEX, 0))
+		r->settings.core_multi_pack_index = 1;
 
(Moving this to group with other no-dir settings.)

I think what you're really trying to do is

	if (r->gitdir)
		(Load settings that require a repo)

	(Load settings that work without a repo.)

I think that you'd be better off extracting the two types of config
into helper methods (prepare_gitdir_settings()/prepare_nodir_settings()?)
across two patches, then in a third removing the BUG and inserting the
if (r->gitdir) above.

Does that seem reasonable?

Thanks,
-Stolee